This invention pertains to ionic hydrocarbon polymers, and more particularly to surface coatings (paint coatings) containing an ionic crosslinking-type polymeric binder system comprising zinc carbonate in combination with a coreactive carboxyl functional polymer.
Protective surface coatings are organic compositions applied to substrates to form continuous films which are cured or otherwise hardened to provide protection as well as a decorative appearance to the substrate. Protective surface coatings ordinarily comprise an organic polymeric binder, pigments, inert fillers and other additives. The polymeric binder functions as a dispersant for the pigments, inerts, and other additives in wet coating compositions and further functions as a binder for the pigments and inert fillers in the cured or hardened paint film. Polymeric binders can be thermoplastic or thermosetting binders based on coreactive components such as a reactive functional polymer adapted to crosslink or coreact with a crosslinking component such as melamine or isocyanate.
Conventional thermosetting polymers often require high temperatures as well as external crosslinkers. Some crosslinkers, such as melamines in conventional industrial coatings or triglycidyl isocyanurate for powder coatings, can cause toxicity problems. Also, the release of volatile by-products, such as caprolactam, from some of these materials can cause film defects, such as cratering and bubbling.
It now has been found that excellent paint coatings can be produced based on an ionomeric polymeric binder comprising a carboxylic acid functional polymer coreacted or neutralized with zinc carbonate. The ionomer or ionic polymer was found to provide interreacting polymer chains which exhibit thermosetting properties comparable to coreactive polymeric binders crosslinked by an amino crosslinker. An ionomer can be defined as a polymer composed of a polymeric backbone containing a small amount of pendant carboxylic acid groups, usually less than 15 mole percent, which are neutralized partially or completely with zinc carbonate to form an ionomer. These ionic moities and their interactions dominate the behavior of the polymer itself where it is believed that the zinc ion of zinc carbonate is exchanged for a hydrogen ion of the polymer carboxyl group. Ionic hydrocarbon polymers for elastomers or plastics are disclosed in U.S. Pat. No. 3,264,272.
In commonly assigned Ser. No. 397,28, filed Aug. 23, 1989, ionomers are disclosed based on certain organic zinc salts having a pka above about 3.0, preferably above 3.8 and active to neutralize certain carboxyl polymers including acrylic copolymers, polyester-acrylic graft polymers, polyester polymers and urethane polymers to provide an ionic thermosetting binder used in surface coatings. Similarly, commonly assigned Ser. No. 397,279, filed Aug. 23, 1989 discloses ionomeric polymers for surface coatings based on an epoxy-ester carboxyl polymer activated by said organic zinc salts as well as zinc carbonate.
In accordance with this invention, changes in the properties of polymeric binders for paint coatings can be achieved through the interaction of zinc carbonate with carboxyl functional polymers where modification of polymer properties is believed due to aggregation of ions described as ionic crosslinking. In this regard, the coreaction of two ion pairs on adjacent polymer chains results in a four-centered aggregate which behaves essentially like a crosslink. Among the dramatic effects that have been observed with paint compositions are increases in the moduli, increases in glass transition temperatures, and increases in viscosities. The formation of ionomeric clusters in protective surface coatings causes the carboxylic acid functional polymers to behave like a phase-separated block copolymer. The use of ionomers in powder coatings specifically is particularly advantageous since it allows control of the process such that the temperature during extrusion is lower than that of the volatilization temperature of the counterion (in the acid form). Thus, the ionomeric links are not formed until after processing of the powder coating, application to a substrate, melting of the powder, flow out, and leveling to form a smooth continuous film. The actual cluster formation takes place during the bake cycle which comprises temperatures higher than the temperature where the salt will lose its acidified counterion. The ionomeric formations can be observed in solvent coatings or high solids coatings as well.
The unique properties of ionomers offer an alternate curing mechanism for solvent and high solids coatings generally and powder coatings especially, with potential for lower baking temperatures, less films defects, a unique balance of physical properties, less toxic curing chemistry, thermal reversibility and lower cost. Thus, the addition of low levels of zinc carbonate to acid functional polymeric binders has been found to have dramatic effects on the physical properties of the polymeric binder. Incorporation of zinc carbonate, for instance, causes a thermoplastic acrylic to exhibit properties such as solvent resistance and hardness comparable to a cured crosslinked paint film. Viscometry has also supported the formation of ionomeric domains in carboxylic acid functional systems, but with ionic crosslinking, these properties are thermally reversible. Useful carboxyl polymers characteristically exhibit low polarity, high hydrophobicity, and low hydrogen bonding characteristics. Zinc carbonate ionomers generally resist water or humidity and produce coatings which are not water sensitive.
The use of zinc carbonate as a neutralizing salt for inducing ionomeric interaction with certain carboxyl functional polymers was surprising because of the high basicity of the carbonate species relative to organic zinc compounds. In this regard, the highly basic carbonate anion would be expected to react with the carboxyl functional polymers prematurely during processing. However, it was unexpected to discover that zinc carbonate was a suitable metal salt for use in ionomeric coatings, and especially ionomeric powder coaings. Also, the use of an organic zinc compound ordinarily requires that the organic zinc salt melt within a temperature range below typical curing temperatures. Zinc carbonate, however, begins to degrade by loss of carbon dioxide at temperatures above about 300.degree. C. which is considerably above curing conditions encountered during the processing and heat cures of paint and protective surface coatings. Thus, the formation of highly desirable paint film integrity properties was very surprising based on using zinc carbonate as the neutralizing agent in a carboxyl functional polymeric system capable of forming ionomers. Further, it has been found that zinc carbonate provides an advantage over various organic zinc compounds where most species evolve acidic volatiles during the curing cycle which can be malodorous and cause corrosion to heat curing equipment. In contrast, zinc carbonate evolves only harmless carbon dioxide and water. When zinc carbonate is used as the neutralizing compound, ionomeric clustering is evidenced by cured film properties exhibiting increased hardness, flexibility, gloss, and solvent resistance. Differential Scanning Calorimetry (DSC) demonstrates an increase in Tg with increasing amounts of neutralizing zinc carbonate indicating that performance properties and processing can be controlled by the extent of neutralization. These and other advantages of the invention will become more apparent from the detailed description and the illustrative examples.